This application is a continuation-in-part of application Ser. No. 07/276,326 filed Nov. 18, 1988.
The invention concerns a process for the conversion of calcium sulfate dihydrate from flue gas desulfurization units (FGD-gypsum) into alpha-hemihydrate (alpha-gypsum), in a salt solution used as a calcinating medium.
For the elimination of sulfur dioxide from flue gases of power plants, two processes among different alternatives are predominantly adopted which are characterized by the use of calcium (hydroxide or of calcium carbonate as a source of calcium ions. The last one is prevailing at new installations, for the reason that the operational costs are lower and the quality of the by-product gypsum is more consistent. The present invention is based on the carbonate process.
Calcium carbonate is abundant in nature in the form of limestone. Though not desired, because of its low reactivity, it always contains at least traces of magnesium carbonate respectively dolomite (CaMg(CO3)2).
Hard coal always contains traces of chlorides, which are decomposed during firing in oxide and volatile hydrochloric acid (HCl). HCl reacts with the carbonate, forming chlorides of the present cations. Being highly soluble, the chlorides are enriched and so they are the limiting factor in recycling the scrubbing liquid. Simultaneously the soluble salts of magnesium are enriched. Regularly the concentration of all dissolved salts in the scrubbing liquid must not surpass 7% and is held in practical operation in the order of 5% by weight.
As a consequence, the precipitated calcium sulfate dihydrate must be not only dewatered, e.g. by a centrifuge, but washed when it will be used in a conventional manner as raw material for all kinds of building plaster.
However, especially in the case of very large power plants, the huge amount of gypsum produced as a by-product often cannot be absorbed by the manufacture of building materials. Piling up at disposals is considered more and more as a danger for the environment.
Additionally, environmental requirements do not permit the dumping of residual scrubbing liquid everywhere. In many cases there is a need to concentrate it by mechanical and thermal treatment to convert it into solid form. That treatment can be done step by step, supplying intermediate brines of different concentration.
A logical way out of those problems can be a re-use of the FGD-gypsum in the coal mine, where it comes from originally. For deep mining, it is an accepted state of the art, that of all kinds of binders on the base of calcium sulfate, Alpha-Gypsum is the best choice. It provides the best mechanical resistances in a short time (sometimes critical for use in mines) and it is not sensitive to elevated temperatures as it is anhydrite.
To convert dihydrate into alpha-hemihydrate by hydrothermal crystallization, it is a known process to subject washed dihydrate essentially free of chlorides to hydrothermal treatment in sulfuric acid solution or in autoclaves.
Further, from DE-OS 31 19 749 it is known to convert the FGD-gypsum by an autoclave treatment without previous washing, that is, in the presence of a scrubbing solution containing approximately 5% by weight of alkali-earth chloride, mainly calcium chloride. The alpha-gypsum obtained by that method is especially suited for use in underground mining, since the efflorescence caused by the chloride contents does not matter and, according to that specification, the mechanical characteristics which are more important are favorably influenced.
However, the conversion of dihydrate containing chloride cannot be done in a sulfuric acid solution since hydrochloric acid would be generated during contact with chloride. Therefore, in the known process, they use conventional autoclaves.
However, the conversion in autoclaves under pressure represents a major investment in machinery and consumes much energy. Additionally, that type of installation requires a relatively large maintenance effort. Altogether conversion with autoclaves is expensive.